1. Field of the Invention
This invention relates to detecting arcing faults in electric power systems utilizing the light emitted by the arcing even in the presence of other light sources. More particularly, it relates to such apparatus and method energized by the emitted light without external power and uses a low-cost optic fiber for communication to central electronics.
2. Background Information
Electric power systems incorporate switches for control and protection purposes. Distribution systems which form part of the overall electric power system include main and branch power buses and circuit breakers mounted in metal cabinets to form switchgear. Interruption of current flow in the buses of the distribution system by a circuit breaker creates an arc as the contacts of the circuit breaker open. These arcs caused by interruption are contained and extinguished in the normal course of operation of the circuit breaker.
At times, however, unintended arcing faults can occur within the switchgear cabinets, such as between the buses, or between a bus and a grounded metal component. Such arcing faults can produce high-energy gases which pose a threat to the structure and nearby personnel. A common approach to protecting personnel from arcing faults in switchgear has been to design the metal enclosures to withstand the blast from the arcing fault. This has been done at great additional cost due to the heavy gauge metal used and numerous weld joints needed to prevent flying debris. Even with these precautions, the blast from an arcing fault inside the switchgear cannot always be fully contained.
Recently, methods have been developed for minimizing the severity of the blast from an internal arcing fault. These methods include pressure sensing and light detection which sense the arcing fault within the switchgear and cause a circuit breaker to trip before significant damage can result. The pressure sensing method is limited by the insensitivity of the pressure sensors. By the time cabinet pressure has risen to detectable levels, the arcing fault has already caused significant damage. The light detection methods used to date are not selective so that any light signal can trigger the sensor. Thus, a flashlight used by service personnel, or sunlight or a photoflash can false trigger the optical sensor. In order to avoid such false tripping of the optical sensors, it has been proposed that optical sensing be combined with current sensing either directly or by sensing the magnetic field created by the current. Unfortunately, these methods are costly and have not proved to be reliable.
U.S. Pat. No. 6,229,680 discloses apparatus and a method for optically detecting arcing faults in electric power systems in the presence of other light sources. Light from components in the electric power system is gathered and split into two light beams. The first beam is passed through a first narrow band filter to extract a wavelength characteristic of arcing in the material, such as copper in switchgear conductors. The second beam of light is passed through a second filter having a narrow bandwidth, not including the characteristic wavelength. The extracted light signals are converted to a sensed light electrical signal and a background light electrical signal, respectively, by photodiodes and compared. If the sensed light electrical signal exceeds the background light electrical signal by a selected threshold, an output device such as a trip solenoid is actuated. While effective, this arrangement requires a bifurcated optic fiber bundle with a separate pair of optic fibers for each component monitored. These fiber optic cables are fragile and expensive. In addition, the optics at the light-gathering end of the optic fibers provide a limited field of view. An alternative arrangement placing the photodiodes close to each of the components to be monitored requires power to be distributed to each of these photo detectors and also provides a narrow field of view.
There is a need for an improved apparatus and method for detecting arcing in electric power system components which is economical and does not require expensive special optic fibers, and which preferably, is self-energized by the arcing, thereby eliminating the need for distribution of power.
The present invention provides an improved apparatus and method for detecting arcing faults in electric power distribution systems even in the presence of other sources of light. A sensor unit includes first photovoltaic means that is responsive to incident light in a first wavelength band including the predetermined wavelength of light produced by an arcing fault to generate a sensed light electrical signal. Second photovoltaic means of the sensor unit generates a background light electrical signal from incident light including light from the component in a second wavelength band not including the predetermined wavelength. Circuit means connects the first photovoltaic means and the second photovoltaic means to an electrically activated light-emitting device with the sensed light electrical signal and the background light electrical signal in opposition to activate the light-emitting device to emit a light signal when the sensed light electrical signal exceeds the background light electrical signal by a selected threshold amount indicative of an arcing fault. A response unit includes an optic fiber transmitting the light signal from the light-emitting device of the sensor unit, and photoelectric means responsive substantially only to the light signal transmitted by the optic fiber for generating an arcing signal in response to the light signal. As the optic fiber is primarily transmitting a digital light signal and providing electrical isolation between the sensor unit and the output arcing signal, it can be a low-cost device. Furthermore, the sensor unit which can be duplicated throughout the electric power system is self-energizing thereby eliminating the need for the distribution of activating power. In addition, the photovoltaic means have a wide field of view thereby eliminating the need for optics to ogather light, or altematively, reducing the number of sensor units required.
Each of the photovoltaic means can comprise at least one photovoltaic cell and filter means, the first photovoltaic means having a filter with a passband equal to the first wavelength band containing the predetermined wavelength generated by an arcing fault, and the filter for the second photovoltaic means having a passband equal to the second wavelength which does not include the predetermined wavelength of an arcing fault.
In an alternative embodiment, a bias electrical signal is added to the signal generated by the first photovoltaic cell. This bias electrical signal can be generated by one or more additional photovoltaic cells which add their electrical output to the electrical output of the first photovoltaic cell so that the light-emitting device is biased towards its operating point, and therefore, the size or number of filtered photovoltaic cells can be reduced to reduce costs.
The method in accordance with the invention comprises extracting from light from the component, light in a first wavelength band including the predetermined wavelength generated by an arcing fault and applying the light in this first wavelength band to a first photovoltaic cell to generate a sensed light electrical signal. Light in a second wavelength band not including the predetermined wavelength is also extracted from light from the component and applied to at least one second photovoltaic cell to generate a background light electrical signal. The background light electrical signal is subtracted from the sensed light electrical signal and a detector light signal is generated when the sensed light signal exceeds the background light electrical signal by a predetermined threshold amount. A photoelectric device is exposed to the detector light signal to generate an arc fault signal.